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COCHANNEL INTERFERENCE REDUCTION IN GSM:

Introduction to Cellular Systems:

Solves the problem of spectral congestion and user capacity. Offer very high capacity in a limited spectrum without major technological changes. Reuse of radio channel in different cells. Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region. Introduction to Cellular Systems

Frequency Reuse:

Each cellular base station is allocated a group of radio channels within a small geographic area called a cell. Neighbouring cells are assigned different channel groups. By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells. Keep interference levels within tolerable limits. Frequency reuse or frequency planning seven groups of channel from A to G Frequency Reuse

Frequency Reuse:

Frequency Reuse

Slide 5:

Co-channel Interference and System Capacity To reduce co-channel interference, co-channel cell must be separated by a minimum distance. When the size of the cell is approximately the same co-channel interference is independent of the transmitted power co-channel interference is a function of R: Radius of the cell D: distance to the centre of the nearest co-channel cell

Slide 6:

For a hexagonal geometry Q = D/R = √3N A small value of Q provides large capacity A large value of Q improves the transmission quality - smaller level of co-channel interference A trade-off must be made between these two objectives

Reducing cochannel interference:

Increasing the separation between two cochannel cells Using directional antennas at the base station lowering the antenna heights at the base station . The use of directional antennas in each cell can serve two purposes : - further reduction of cochannel interference - increasing the channel capacity when the traffic increases. Reducing cochannel interference

Cell Sectoring:

Cell Sectoring

Cause of Cochannel Interference in sectored antenna:

Cause of Cochannel Interference in sectored antenna

Notch in sectored antenna:

The interfering site could cause interference at those cells within a 19◦ sector in front of the cell. In an ideal situation the antenna pattern of the serving cell must be rotated clockwise by 10◦ such that the notch can be aimed properly at the interfering cell Notch in sectored antenna

Phase shifting network in electrical tilting:

Vertical antenna pattern:

Horizontal antenna pattern:

Cautions in Tilting Antennas:

Elements playing a major role in justifying antenna tilting- The antenna vertical pattern The antenna height Cautions in Tilting Antennas

Umbrella pattern:

For controlling the energy in a confined area, the umbrella-pattern antenna can be developed by using a monopole with a top disk The size of the disk determines the tilting angle of the pattern. The smaller the disk, the larger the tilting angle of the umbrella pattern. Umbrella pattern

Umbrella pattern:

Umbrella pattern

Benefits of umbrella pattern:

Energy is confined to the immediate area of the antenna Reduces cochannel and long-distance interference. We can increase antenna height still reduce cochannel interference The frequency-reuse distance can be shortened Benefits of umbrella pattern

Microcell Zone Concept:

• Antennas are placed at the outer edges of the cell • Any channel may be assigned to any zone by the base station • Mobile is served by the zone with the strongest signal. • Handoff within a cell – No channel reassignment – Switch the channel to a different zone site Reduce interference – Low power transmitters are employed Microcell Zone Concept

Microcell Zone Concept:

Microcell Zone Concept

Power control for reducing interference:

Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel Can be controlled only by mobile station – long battery life – increase SIR Power control for reducing interference

Use of Parasitic element:

Creating a desired pattern in a certain direction. currents appearing in several parasitic antennas are caused by radiation from a nearby drive antenna . Ways to combine a driven antenna & a single parasite : - Normal spacing - Close spacing Use of Parasitic element

Normal spacing:

Close spacing:

Length of both elements same - Forms a null at the centre - D irective gain of 3 dB relative to a single element Length of parasite greater than active element - Parasite acts as reflector - D irective gain of 6 dB Length of parasite less than active element - Parasite acts as director - D irective gain of 8 dB Close spacing

Close spacing:

Close spacing

JOINT DEMODULATION OF CO CHANNEL SIGNALS:

JOINT DEMODULATION OF CO CHANNEL SIGNALS Dominant interfererence likely exists in GSM Number of nearby co channel interference is rather small Cancellation of it can improve the receiver performance It depends on the dominant to rest of interference ratio DIR.